The increase in the printing speed of rotary offset machines and the cutting of a web of material into sheets even half the size of the printing-cylinder-generated surface make it necessary for the machine operators to change the stack manually so often as to have an adverse effect on the continuous supervision of the printing machine.
In a prior art sheet deliverer of the kind specified, for example, in DE-AS 27 51 489, therefore, a triangular pusher and a rectangular pusher are provided, which are inserted manually at the stack-end face to separate an upper portion of the stack. A parting line is first formed manually for the tip of the triangular pusher, which is inserted first, so as to facilitate its insertion and avoid damage to the front edges of the sheets.
To separate component layers from a stationary stack it is known from DE-OS 36 19 676 to arrange at one edge of the stack a sheet-turning device which, when a component layer is reached, forms a gap at the edge which facilitates the introduction of separating heads. The separating heads are disposed on carriages and can travel along the end face. According to that method, the separating heads can displace the component stack which they seize in relation to the subjacent stack. With this prior art method it is impossible to change the stack without interrupting the deposition of the sheets.
Lastly, an apparatus is known for changing a stack in a sheet deliverer from DE-OS 29 42 965, which comprises a rake which can be driven in the conveying direction of the sheets between the top side of the stack and the sheets to be deposited thereon. To enable this operation to be performed without damaging the sheets, the front side of the rake has blowing nozzles. However, such a device is unsuitable for separating already stacked sheets, merely because there is no guarantee that the individual tines of the rake will be introduced between the same sheets. Nor can the tines be introduced into the stack at the end face without damage. The air jets emerging from the tines are unable to form a clearly defined gap between the same sheets over the whole end face.
It is an object of the present invention, therefore, to provide an apparatus for changing a sheet stack formed on a conveying element in a sheet deliverer, by means of which a change can be made to the stack rapidly, without manual assistance, and without the risk of damage to the sheets.